1. Field of the Invention
The present invention relates to a hybrid vehicle control apparatus that controls an engine that rotates at least one first wheel and an electric motor that rotates at least one second wheel that is mechanically independent from the engine. More particularly, the present invention relates to stabilizing the vehicle driving performance when drive forces are obtained from both the engine and the electric motor.
2. Background Information
A four-wheel drive type vehicle control apparatus is well known that drives both the first wheels (e.g., rear wheels) and the second wheels (e.g., front wheels) with an engine. This kind of traditional 4WD system uses a transfer mechanism that includes a multiple disk clutch mechanism to distribute the engine drive torque between the front and rear wheels. Consequently, the total drive torque is held constant even if the distribution ratio of the drive torque changes.
In recent years a hybrid 4WD system has been proposed (see Japanese Laid-Open Patent Publication No. 8-300965) which rotates one set of wheels with an engine and rotates the other set of wheels with an electric motor. In this system, there is no mechanical connection between the front and rear wheels. Rather, the drive torque that is transmitted to each set of drive wheels must be adjusted relative to each other in order to keep the total drive torque constant. This presents new problems in terms of driveability.
In view of the above, there exists a need for an improved hybrid vehicle control apparatus that overcomes the problems of the prior art. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
Generally, the electric motor is advantageous over the engine in terms of response but disadvantageous in terms of output. As a result, several problems arise. For example, as previously mentioned, it is preferable from the standpoint of vehicle driving performance to adjust each drive torque such that the total drive torque of the front and rear wheels is constant. However, if this is done, there is the potential problem that a feeling of speed loss and/or degraded maneuverability will occur when the output requested of the electric motor for rotating the non-engine driven wheels exceeds the output limit of the electric motor. Furthermore, an extremely large output is required of the electric motor if this hybrid 4WD system is to achieve the same degree of 4WD traveling region as a traditional engine-only 4WD system.
In view of these circumstances, the object of the present invention is to provide a hybrid vehicle control apparatus that enables stable 4WD travel over a wide range even with an electric motor that is small in comparison to the engine.
Therefore, in accordance with one aspect of the present invention, a hybrid vehicle control apparatus is provided for a vehicle having front and rear wheels with at least one of the front and rear wheels being an engine driven wheel driven by an internal combustion engine and at least one of the front and rear wheels being a non-engine driven wheel driven by an electric motor that is mechanically independent from the engine. The hybrid vehicle control apparatus basically comprises a target motor torque setting section, a motor output limit determining section, and a torque increasing section. The target motor torque setting section is configured to set a target drive torque of the electric motor. The motor output limit determining section is configured to determine if the target drive torque of the electric motor reaches an output limit of the electric motor under current operating conditions. The torque increasing section is configured to increase drive torque to the engine driven wheels when it is determined that the target drive torque of the electric motor reaches the output limit of the electric motor.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.